1. Field of the Invention
This invention relates to the field of electrical switches and particularly to the field of remotely controlled electrical switches capable of interrupting the application of a source voltage to a load in response to an overload condition.
This invention relates more particularly to the field of solid state power controllers or solid state electrical switches such as solid state relays capable of detecting overload conditions and being able to interrupt the electrical service to the load automatically thereby preventing further damage and having the additional capability of being removely resetable after clearing the overload condition. Typical applications for the invention electrical switch include marine and aircraft as well as industrial applications requiring noise free closure and interruption of source power, compact size, extended switch life, low power dissipation and high reliability.
2. Description of the Prior Art
Electromechanical switching devices using solenoid driven means to transfer electromechanical contacts to apply a source voltage to a load, such as relays are well known. When coupled with an electromechanical circuit breaker, a relay provides a remotely controlable power control function. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure thereby afording low power dissipation. As used in a-c power control applications, mechanical contacts cannot conveniently be timed to apply power to the load as the source voltage crosses zero voltage, or to interrupt the application of power as the load current passes through zero so as to minimize transient disturbances. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.
The solid state relay is known to overcome the problem of applying a source voltage to a load as the source voltage crosses zero voltage. Solid state relays are also able to interrupt load current as the load current crosses zero. However, solid state relays introduce a substantial voltage drop at the point of circuit closure, thereby providing relatively high power dissipation in comparison to electromechanical switching devices. For this reason solid state relays find their greatest application in controlling electrical service to loads requiring relatively low load currents.
Circuit breakers used to protect the electrical service for a load are typically complex mechanical devices that rely on magnetic circuits or thermally actuated means to detect an overload condition. Environmental effects such as vibration, moisture, temperature and age along with operational effects such as thermal and mechanical fatigue from past overload cycle experiences contribute to the degradation of these assemblies. Contact arcing that is sustained by a high voltage source as the contacts part to protect a circuit experiencing overload conditions, contribute to circuit breaker contact degradation.
The invention electrical switch couples the preferred features of an electromechanical switching means of providing a remotely controllable switch closure having a low voltage drop and the preferred features of a solid state relay of first applying a source voltage to a load as the source voltage crosses zero voltage, and later interrupting the current path as the load current crosses zero with the function of a remotely resetable solid state circuit breaker, absent those features of an electromechanical circuit breaker that contribute to circuit breaker degradation. In addition, the invention electrical switch can be easily fabricated to have load current limitations that far exceed the customary practical limits for solid state relays.